Molded electric heater and method of making same



Aug. 15, 1961 v, TUTTLE 2,996,594

MOLDED ELECTRIC HEATER AND METHOD OF MAKING SAME Filed May 18. 1956 3 Sheets-Sheet l INVENTOR. v

M \Q 20 a h Aug. 15, 1961 v, TUTTLE 2,996,594

MOLDED ELECTRIC HEATER AND METHOD OF MAKING SAME Filed May 18, 1956 3 Sheets-Sheet 2 85 64 pg 1118! g! 1 fi wm ww D. V. TUTTLE Aug. 15, 1961 MOLDED ELECTRIC HEATER AND METHOD OF MAKING SAME 3 Sheets-Sheet 3 Filed May 18. 1956 8 my W.

a m a United States Patent 2,996,594 MOLDED ELECTRIC HEATER AND METHOD OF MAKING SAME Daniel V. Tuttle, Kirkland, Ill., assignor to Tuttle Electric Products, Inc., Kirkland, 111., a corporation of Illin'ois Filed May 18, 1956, Ser. No. 585,653 14 Claims. (Cl. 219- 19) This invention relates to an electric heater in which the heating resistance is supported in an exposed manner by a molded body of refractory electrical insulating material, and has reference more particularly to such heater, and to the method of making same, with the body molded onto the resistance.

Heretofore electric heaters with exposed heating resistances have usually been. made either by molding a resistance supporting body of refractory material with grooves or other facilities for attachment of the resistance thereto and thereafter attaching the resistance thereto, or by providing a metal frame with spaced apart insulators of refractory material through which the resistance is threaded to support the resistance and electrically insulate it from the frame.

Such electric heaters not only require costly equipment such as molds particularly designed for forming the refractory parts to accommodate and retain the resistance in place, or mechanisms for producing the particular form of metal frame required, but they also involve considerable assembling expense. Moreover, the construction of such previous heaters oftentimes did not alford maximum heating eificiency for the purpose for which they were designed to be used.

In accordance with the present invention the resistance is merely located in position in a mold in which the body of refractory material is shaped with the resistance securely and permanently embedded therein, and the mold employed is of simple and inexpensive construction utilizing as a part thereof an outer shell that remains attached to the molded body and becomes the outer shell of the heater and provides the mounting facilities by which the heater is installed in the place of use, thereby providing in the single molding operation a unitary heater complete with resistance, refractory body and heater protecting and mounting shell.

The principal objects of the invention are to provide an improved exposed resistance electric heater; to simplify the construction of such heaters and minimize the cost thereof; to eliminate expensive molds or equipment and assembling operations; to improve the efiiciency of such heaters; and to provide an exposed resistance electric heater which is more conveniently adaptable to the various purposes for which such heaters are generally used, these and other objects being accomplished as pointed out more particularly hereinafter and as shown in the accompanying drawing in which:

FIG. 1 is an end view of a heater constructed in accordance with the present invention;

FIG. 2 is a side view of the heater of FIG. 1 with a large portion thereof at one end and a small portion at the other end shown in section, both on the line 2-2 of FIG. 1;

FIG. 3 is an enlarged fragmentary cross sectional view of the heater of FIG. 2 taken on the line 3-3 thereof;

FIG. 4 is a sectional view on the line 4-4 of FIG. 3;

FIG. 5 is a sectional view on the line 5-5 of FIG. 3;

FIG. 6 is a vertical sectional view of a typical clothes dryer showing the heater of FIGS. 1 and 2 installed therein;

FIG. 7 is a rear view of the dryer of FIG. 6 with the rear wall removed;

' therein in the manner indicated at FIG. 8 is a vertical sectional view with an intermediate part broken away of the heater of FIGS. 1 and 2 in the mold in which it is produced;

FIG. 9 is a top view somewhat reduced in size of the mold of FIG. 8;

FIG. 10 is a sectional view on the line 10-10 of FIG.

FIG. 11 is a fragmentary view showing a portion of the core of the mold and the manner of applying the resistance thereon preparatory to the molding of the heater;

FIG. 12 is a side view with a portion broken away of the heater of FIG. 1 with the terminals at one end there- 01;

FIG. 13 is a side view with a portion broken away of a modification of the heater of FIGS. 1 and 2;

FIG. 14 is a central sectional view of another modified form of heater which may be made in accordance with the present invention; and

FIG. 15 is a side view partly in section of a jacketed form in which the heater may be made.

Referring to the drawings, and particularly to FIGS. 1 to 5 inclusive, which show a preferred embodiment of the invention, the reference numeral 20 indicates a thin walled cylindrical tube, preferably of metal, which constitutes the outer wall of the heater and 21 indicates a thick cylindrical lining of molded refractory material which is preferably molded in place in the tube 20 and thereby secured thereto. This lining 21 preferably terminates short of both ends of the tube 20 as shown in FIG. 2 leaving an unlined portion 22 of the tube 20 projecting beyond the refractory lining 21 at each end, and the lining 21 is preferably provided with a circumferential series of close adjoining parallel ventilating openings 23 extending therethrough from end to end.

The heating element, which is indicated at 24, is a cylindrical spiral winding of bare electrical resistance wire preferably energizable to a glowing heat and has the coils 25 thereof spaced apart as shown so that there is a substantial intervening space between the successive coils thereof, and the winding which extends from end to end of the refractory lining 21 has each of the coils 25 correspondingly embedded in the inner face of the lining to a depth to be safely retained therein but with a large portion of the interior face of each coil exposed substantially as shown in FIGS. 2. and 4.

The winding 24 is embedded in the lining 21 by molding the latter in contact with the winding and thus the inner face of the lining is formed with a spiral groove 26 in which the winding 24 and each individual coil 25 thereof is closely confined and retained against displacement.

For additionally securing the coils 25 in place, the lining 21 is formed on the inner face at suitable circumferential intervals with ribs 27 which extend from end to end of the lining, four such ribs being shown in the illustrated embodiment, and these ribs completely cover the coils 25 at the places where they extend thereacross, as shown in FIGS. 1, 2, 3 and 4 so that the coils at the rib locations are securely anchored to the lining 21.

Each end coil 25 of the resistance terminates in a portion which extends into the lining 21 to greater depth 30 in FIG. 1 and is continued outwardly from that place through and projects from the respective end of the lining 21 for connection of current supply conductors thereto, such projecting portions being indicated respectively at 3'1 and 32.

The current supply conductors may be connected directed to these projecting end portions 31 and 32 of the winding 24, but it is preferred to provide the opposite unlined end portions 22 of the cylindrical shell 3 with attached terminal posts 33 and 34 respectively to which the projecting end portions 31 and 312 of the resistance 24 are respectively connected.

Each post 33 and 34 is preferably located in the immediate vicinity of the place where the respective end portion 31 or 32 of the resistance projects from the end of the lining 21 and projects radially outward through an opening in the respective unlined end portion 22 of the tube 20, and if the tube 20 is of metal each post 33 and 34 is electrically insulated from the respective end portion 22 thereof in any convenient manner.

For example, each terminal post may comprise a metal stem 36 extending through inner and outer porcelain parts 37 ad 38 respectively with the inner part 37 provided with a sleeve portion 39 which encircles the stem 36 and projects through the opening 35 into a central recess 40 of the part 38, the stem 36 being provided at the inner end with a head 41 between which and the inner porcelain part 37, a loop 42 of the respective end por tion 31 or 32 of the resistance 24 is clamped by a nut 43 on the outer threaded end of the stem 36, the said nut serving also to clamp the porcelain parts 37 and 38 onto the respective unlined end portion 22. An extra nut 44 is provided on the outer threaded end of the stem 36 for securing a supply conductor to the stem 36.

Thus the conductors for supplying current to the resistance "24 may be connected at the exterior of the tube 20 to the outer ends respective of the stems 36 of the posts 33 and 34.

When the heater above described is energized, the place of greatest heat intensity is at the inner side of the lining 21 and concentrated in the central opening of said lining 21 and the heat is uniformly distributed throughout the length of said central opening as each coil 25 of the resistance 24 is a source of heat and exposed at its inner side to the interior of the central opening of the lining 21, and if air or other gas is passed through the central opening of the lining 21 it is heated throughout to a high temperature in its passage therethrough.

The lining 21 necessarily becomes heated and air or gas passing through the longitudinal openings 23 of the lining tends to absorb the heat transmitted to the lining 21 and provide an extra supply of heated air or gas.

While the heater above described may be used for various purposes, as for example, as a kiln, it is usable to particular advantage in clothes dryers or the like, as for example, as shown in FIGS. 6 and 7 in which a popular form of domestic clothes dryer is illustrated.

In said FIGS. 6 and 7 the dryer comprises an outer cas ing composed of two side walls 45 and 46 with top and bottom walls 47 and 48 respectively and front and rear walls 49 and 50 respectively, and the interior of the casing is divided by a combined vertical partition 51 and a horizontal partition 52, both of which extend from side to side of the casing, into a rear compartment space 53 and a bottom compartment space 54 which communicate with one another.

At the front of the partition 51 and above the partition 52 is a relatively large compartment space or drum chamber 55 occupied by a drum 56 in which clothes are dried and this drum has a central spindle 57 at the rear end journaled in a bearing 58 on the partition 52 to rotate on a horizontal axis and at the rear of the partition 51 the spindle 57 has a large sheave 59 fixed thereon to which power is supplied from a motor 66 through belts 61 and 62 to and from sheaves 63 and 64 on an intermediate countershaft 65 to rotate the spindle 57 and the drum 56 at a relatively low rate of speed.

The drum chamber 55 is separated from the front Wall. 49' of the casing by a partition 66, to which the forward end of the partition 52 is attached, and provides a chamber 67, hereinafter referred to as the exhaust chamber, at the front of the casing and this partition 66 has an opening therethrough through which the forward end of 4 the rotatable drum 56 projects and by which the drum is quite closely confined.

The front end wall of the drum has a large central opening to accommodate the inner end of a large thirnble 68 about which the drum front wall rotates, and this thirnble is secured to the front wall 49 in a position concentric with the drum 56 to afford access to the interior of the drum from the front of the dryer, the opening through this thimble being closed at the front end by a door 69 on the front wall 49.

The rear wall of the drum 56 is provided throughout the area thereof with air inlet openings 70 and the front wall of :the drum is likewise provided with air outlet openings 71 throughout the area thereof, and the back wall of the drum is spaced sufticiently from the vertical partition 51 to provide ample access to air to the rear wall openings 70.

In the operation of this dryer the clothes to be dried are located in the drum '56 which is rotated by the motor 60 to tumble the clothes in the drum, the latter being provided on its peripheral wall with ribs or formations 72 to assure the required tumbling operation, and during the drying operation heated air is circulated through the interior of the drum from the rear openings 76 to the front openings 71 of the drum to dry the turnbling clothes.

To supply the heated air for drying, the vertical partition 51 is provided with an opening therethrough, above the drum, in which a heater 73, such as above described and shown in FIGS. 1 and 2 is mounted, as for example by a flanged ring 74 or circumferentially spaced angle brackets on the exterior of the cylindrical shell 20 of the heater which are secured to the partition 51, so that the rear chamber 53 of the casing communicates with the chamber 55 in which the drum 56 is located, solely through the central opening of the heater and the long openings 23 in the lining 21 thereof.

The bottom wall 43 of the casing is elevated above the floor on which the dryer is supported and has a large opening 75 therethrough through which air is supplied to the chamber 54 and therefrom to the chamber 53.

A blower 76, operable by a motor 77, is mounted on the bottom wall 48 of the casing at the rear end of the chamber or compartment 54 with the blower outlet 78 projecting through the rear wall 50 of the dryer casing and a duct 79 is connected at its front end to the partition 66 and leads from the chamber 67 to the intake of the blower 76 so that in the operation of the motor 77 and blower 76 air is exhausted from the chamber 67.

The rear end of the heater 73 is spaced suificiently from the rear wall 50 of the dryer to afford ample space for free entry of air into the rear end of the heater, the rear wall 50 preferably having a relatively large heat reflector lining on the inner face thereof opposite the rear end of the heater 73, and in the operation of the dryer the blower 76 draws a large volume of air into the compartments 54 and 53 through the opening 75 and from the chamber 53 through the central opening of the heater 73 and through the opening 23 thereof into the drum chamber 55, thereby heating the air to a high temperature in its passage through the heater 73, and the thus heated air which surrounds and heats the drum 56, is drawn by the blower 76 through the openings 70 in the rear wall of the drum and circulates through the tumbling clothes in the drum to extract moisture therefrom, and the moisture laden air is exhausted from the interior of the drum through the openings 7i in the front Wall of the drum into the chamber 67 from which it is discharged to the exterior of the dryer through the duct 79 and the blower 76.

A large volume of highly heated air is required for effectively drying the clothes and in the desired minimum time, and a large amount of current is usually required for the purpose, and I have found that a heater 73 constructed and arranged as shown and describec' herein dries the clothes more completely and in less time and with considerably less current consumption than with heaters heretofore employed for the purpose.

The heater of FIGS. 1 to 5 inclusive may be conveniently made with a simple mold as shown in FIGS. 8, 9, l0 and 11 wherein 81 indicates a base on which a cylindrical core 82 is supported in an upright position and is of a diameter somewhat less than the diameter of the central opening through the lining 21 of the heater, for reasons hereinafter explained, the core 82 having a cylindrical enlargement 83 at the lower end by which the core is located at an elevation above the base, and this enlargement preferably has a ring 84 therearound which has a slip fit on the enlargement 83 and is removable therefrom, although the ring 84 may be an integral part of the enlargement 83 if desired.

The ring 84 has an annular shoulder 85 therearound and the portion of said ring above the shoulder 85 is of a diameter to receive therein one end of the cylindrical tube 20 which becomes the outer shell of the completed heater and the length of the portion of the ring 84 above the shoulder 85 corresponds to the desired length of unlined portion 22 of the tube 20 in the finished heater, the arrangement being such that when the cylindrical tube 20 is placed in position on the end of the enlargement 83 and seated against the shoulder 85, the tube 20 is concentric with the core 82 with a wide annular space of uniform width therebetween.

The ring 84 supports a plurality of long cylindrical cores 86 vertically at circumferentially spaced intervals in the annular space between the central core 82 and the cylindrical tube, these cores 86 being of suitable size and location to produce the vent openings 23 in the lining 21, and these cores 86 and the central core 82 are of a length slightly greater than the length of the lining 21 to be provided in the cylindrical tube 20 of the heater and preferably beveled around their upper ends as shown in FIG. 8.

The cores 86 may be permanently attached to the ring 84 or may be individually removable therefrom, as desired, with reduced stem portions 87 at their lower ends which slip into elongated vertical sockets 88 in the ring 84, and while it is indicated above that the cores 86 are cylindrical, these cores may have a slight taper to facilitate drawing thereof from the molded lining 21 which is formed therearound.

The core 82 does not provide a mold surface but instead is a core for supporting a plurality of elongated transversely curved panels 89 which are applicable around the core 82 to form an external generally cylindrical surface thereon, four quarter cylindrical segments or panels 89 being shown herein, although the number thereof may be varied.

These panels or segments 89 are of a length to extend substantially the full length of the core 82 and each has the lateral edges thereof beveled or turned in as indicated at 90 so that when they are placed in lateral edge to edge relation around the core 82, as shown in FIG. 10, the edges of adjoining panels at the outer extremity of the bevel abut or substantially abut against one another and the exterior surfaces thereof constitute conjointly a surface of cylindrical contour with an external V-shaped groove 91 (see FIG. 11) at the juncture of each adjoining pair of panels 89 and extending from end to end thereof.

The panels 89 may be solid segments with the inner faces thereof conforming to the contour of the core 82, but preferably are of stiff sheet metal with the lateral edges bent inwardly as shown particularly at 92 in FIG. to provide the bevel 90 and minimize the bearing contact against the core 8 2, and the core 82 may be solid or a cylindrical tube with plain cylindrical exterior but may be provided with external lengthwise extending grooves 93 to receive the inturned edges 92 of the panels 89 and facilitate the placing of the panels in position on the core 82.

A funnel-like member 94 is preferably provided with a cylindrical lower end portion 95 adapted to fit into the end of the tube 20 and this cylindrical portion 95 has upright Web members 96 therein which support a tapered cap 97 which fits over the upper end of the core 82 and one of the web members 96 is provided with an upright tubular sleeve 98 approximately midway between the wall of the cylindrical portion 95 and the cap 97 for holding a terminal end of the resistance winding 24 in the molding operation.

The annular ring 84 and base 81 also have a small vertical opening 99 therethrough to receive the other terminal end of the resistance winding.

In making the heater with the above described equipment, the panels or segments 89 are assembled around the core 82 as shown in FIG. 10 with their lower ends abutting against the top face of the enlargement 83, as shown in FIG. 8, and a sheet 100 of material, such as wax paper, which will not adhere to the molded lining 21 and is sufficiently flexible to conform to the grooves 91 is wrapped around the assembled panels 89, after which a resistance wire 24 of the required length is wound tightly around the assembled panels 89 over the Wax paper 100 thereon with the coils 25 spaced apart, as

' shown, and an end portion of the resistance wire which is to constitute one of the terminal ends 31 or 32 of the completed heater, for example the terminal end 31, is inserted through the opening 99 of the enlargement 83 and base 81 as shown in FIG. 8.

The ring 84 with the small cores 86 assembled thereon may then be placed in position on the enlargement 8-3, or the cores 86 may be individually assembled on the already applied ring 84, after which the cylindrical tube 20 is placed in position on the ring 84 as shown in FIG. 8, whereupon the funnel member 94 is inserted in the upper end of the tube 20 with the other terminal end portion of the resistance 24, for example, the end portion 32 extending upwardly through the opening of the sleeve 98 as shown in FIG. 8.

A flowable plastic mass of a suitable mixture or compound, such for example as a mixture of steatite, which sets up in a hard solid form in which it withstands high temperatures and is electrically non-conductive, is then poured into the funnel 94 and into the space between the cylindrical tube 20 and the spiral winding 24 and wax paper 100 on the core 82, as indicated at 101 in FIG. 8, up to the level indicated at 102 in FIG. 8 at such distance below the upper end of the tube 20 to leave the desired unlined portion 22 at that end of the tube 20, and tamped in place if necessary, and then allowed to set and harden, after which the funnel member 94 is removed, and also the central core 82, panels 89 and wax paper 100 and also the small cores 86, leaving in the cylindrical tube 20 the molded material 101 which constitutes the lining 21 with the resistance wire 24 embedded therein.

In the molding operation, the plastic mass 101 fills in around the sides of the coils 25 but leaves a substantial width of the inner face, which is covered by the Wax paper 100 in the molding operation, exposed and moreover the plastic mass forces the wax paper down into the grooves 91 between the segments or panels 89 and forms therein the ribs 27 which overlie the coils 25 at intervals therearound.

By reason of the terminal ends 31 and 32 of the resistance being extended through the sleeve 98 and opening 99 in the molding operation, these ends project outwardly from the opposite ends of the molded lining 21 and supply conductors may be connected directly thereto or they may be connected to terminal posts 33 and 34 if desired.

In some cases it may be desired to locate both terminal wires or terminal posts at one end of the heater, in which .7 event a terminal end portion is provided atone end of the lining 21 of said length that it may be bent back as indicated at 103 in FIG. 12 and extended through one of the openings 23 in the lining 21, as indicated at 104, and connected to a terminal post 105 at the same end of the cylindrical tube 20 at which the terminal post 106, to which the other terminal end portion 107 of the resistance 24, is located.

The above described heater may be made with a substantial taper from end to end, as shown in FIG. 13, in which the corresponding parts thereof are identified by the same reference numerals as those of FIGS. 1 to 5 inclusive, and moreover the heater may be made in a shallow conical form such as shown in FIG. 14 in which the reference numeral 108 indicates the lining of refractory material which is formed in flat conical shape in a flat conical pan or shell 109 and the resistance is in the form of a flat conical spiral 110 embedded in the upper exposed face of the lining 108.

In this FIG. 14 form of the heater, a plate 111 of glass capable of withstanding high temperatures may be secured to the upper side of the heater by clips 112 at intervals around the heater to adapt the latter for surface heating purposes, as for example in electric ranges.

Also, the heater may be enclosed in an outer jacket to provide air circulation spaces around the heater.

For example, the heater of FIGS. 1 and 2 may be mounted, as shown in FIG. 15, within a tubular casing 113 to provide an annular passageway around the cylindrical shell 20 of the heater, and for this purpose the shell 20 may be provided near each end and at intervals therearound with arms 114 projecting radially outward therefrom and secured at their outer ends to the tubular casing 113.

Such heater with tubular casing 113 may be advantageously employed, for example in the dryer of FIGS. 6 and 7, by providing the partition 51 with an opening to accommodate the casing 113 of the heater of FIG. 15 and securing the casing 113 containing the heater in that opening in the same manner that the heater alone of FIGS. 1 and 2 is mounted in the partition 51 in FIGS. 6 and 7.

Thus when the heater with the casing 113 is installed in the partition 51 an additional avenue of air supply is provided, between the casing 113 and the heater shell 20, and as the latter becomes quite highly heated when the resistance 24 is energized, especially for a prolonged period of time, greater efiiciency may be obtained.

While I have shown and described my invention in a preferred form, I am aware that various modifications can be made therein without departing from the spirit of my invention, the scope of which is to be determined by the appended claims.

What is claimed is:

1. An electric heater comprising a cylindrical helix of bare heat resistance wire having a series of laterally spaced substantially coaxial coils and a mounting for said coils conjointly therewith defining a passage encircled by said coils, the said mounting comprising a rigid tubular body of electrically nonconductive refractory material having an undivided circumferentially continuous wall surrounding the series of coils having the longitudinal axis of the wire of the coils substantially paralleling the inner face of the circumferentially continuous wall, the wire of each said coil. being embedded in said wall substantially throughout the length of the wire thereof and the wall being irremovably molded to the wire in conformity therewith in intimate gripping engagement with the surface of the wire at opposite sides thereof in matrix relation to the cross sectional contour of the wire with the bare face of the wire at the inner sides of the coils exposed to the passage.

2. An electric heater comprising a cylindrical helix of bare heating resistance wire providing a series of laterally spaced substantially coaxial coils, and a mounting for said coils conjointly therewith defining a cylindrical passage coaxial with and encircled by said coils, the said mounting comprising a rigid tubular body of electrically nonconductive refractory material having an undivided circum ferentially continuous wall surrounding the series of coils and said coils having the longitudinal axis of the Wire thereof substantially paralleling the inner face of the circumferential wall and said wall being inseparably molded to the wire of each coil with the bare face of the wire at the inner sides of the coils exposed to the passage and a blower communicating with one end of the passage to discharge air therethrough crosswise of the wire of the coils and in contact with the bare face of the wire at the inner sides of the coils.

3. An electric heater comprising a heating element which consists of a distributively arranged winding of bare heating resistance wire, and a heating element backing at one side of which the bare wire heating element is embeddedly supported therein in matrix relation thereto in an exposed manner, the said backing being a rigid body of electrically non-conductive refractory material molded in inseparable supporting conformity to the cross sectional contour of the wire of the heating element with the bare face of the wire exposed at the side thereof remote from the backing, the backing being formed with spaced apart ribs having transverse apertures through which the wire of the winding extends.

4. An electric heater comprising a heating element which consists of a distributively arranged winding of bare heating resistance wire, and a heating element backing at one side of which the bare wire heating element is embeddedly supported therein in matrix relation thereto in an exposed manner, the said backing being a rigid body of electrically non-conductive refractory material molded in inseparable supporting conformity to the cross sectional contour of the wire of the heating element with the bare face of the wire exposed at the side thereof remote from the backing, the backing being a thick walled tube and the winding being at and conformed substantially to the inner side of the tube and the wall being formed with a plurality of peripherally spaced passages extending through the wall from end to end of the tube.

5. An electric heater comprising a heating element which consists of a distributively arranged winding of bare heating resistance wire, and a heating element backing at one side of which the bare wire heating element is embcddedly supported therein in matrix relation thereto in an exposed manner, the said backing being a rigid body of electrically non-conductive refractory material molded in inseparable supporting conformity to the cross sectional contour of the wire of the heating element with the bare face of the wire exposed at the side thereof remote from the backing, the backing being a thick walled cylindrical tube externally conformed to and confined within and supported by a cylindrical metal tube and the cylindrical metal tube being contained in another tube with an annular air circulation space between the tubes and open at both ends.

6. An electric heater comprising a heating element which consists of a distributively arranged winding 04 bare heating resistance wire, and a heating element backing at one side of which the bare wire heating element is embeddedly supported therein in matrix relation theretc in an exposed manner, the said backing being a rigic body of electrically non-conductive refractory materia molded in inseparable supporting conformity to the cros: sectional contour of the wire of the heating element witi the bare face of the wire exposed at the side thereof III10tt from the backing, the backing being a thick walled cy lindrical tube externally conformed to and confined with in and supported by a metal tube, the thick walled tub defining a passage which is open at both ends and arounr which the winding is distributively arranged, and a par tition by which the heater is supported for communica-' tion through the said passage thereof from one side of the partition to the other side of the partition, and means for enforcing circulation of air through said passage.

7. An electric heating unit comprising an electrical heating resistance composed of a long bare wire of electrical resistance material, and a rigid body of electrically non-conductive refractory material presenting a face throughout which the long wire is distributively arranged with its longitudinal axis substantially paralleling said face, said wire being irremovably embedded in the refractory material of the body with a substantial amount of the surface of the wire exposed to radiate heat therefrom, the body having portions thereof at opposite sides of the wire conformed to the surface of and gripping the wire therebetween in intimacy of contact with the surface thereof providing permanent unification of the wire with and irremovability of the wire from the body.

8. An electric heater in accordance with claim 7 wherein the side of the wire remote from the body is exposed substantially throughout the length thereof.

9. An electric heater in accordance with claim 7 wherein the conformity of the portion of the body to the surface of and gripping of the wire therebetween in intimacy of contact is substantially continuous throughout the length of the wire.

10. An electric heater in accordance with claim 7 wherein the body is a thick wall at one side of which the wire is distributively arranged substantially in parallelism with the wall throughout the area thereof.

11 An electric heater in accordance with claim 7 wherein the body is a thick concave-convex wall and the winding is at and conforms substantially to the concave side thereof.

12. An electric heater in accordance with claim 7 wherein the body is a thick walled tube and the resistance wire is at and conforms substantially to the inner side of the thick wall of the tube.

13. An electric heater in accordance with claim 7 wherein the body is a thick wall at one side of which the resistance wire is distributively arranged and at the opposite side of which the wall conforms to and is supported by a sheet metal shell.

14. An electric heater as defined in claim 7 wherein the body is a thick walled cylindrical tube externally conformed to and confined within and supported by a metal tube and the electrical heating resistance is a helix with a series of circumferentially extending coils of the resistance wire spaced apart in the direction of the length of the tube.

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